The present invention relates to methods and apparatuses for controlling vacuum pressure in internal combustion engines. More particularly, the present invention pertains to methods and apparatuses for controlling vacuum pressure in internal combustion engines having brake boosters, which use vacuum pressure to improve braking force.
In a typical engine, fuel is injected into an intake port from a fuel injection valve to charge a mixture of fuel and air to the associated combustion chamber. The air-fuel mixture is ignited by an ignition plug to produce torque. This type of combustion, in which air-fuel mixture is ignited in a combustion chamber, is generally called homogeneous charge combustion. In an engine performing homogeneous charge combustion, a throttle valve located in an intake passage controls the cross-sectional area of the intake passage thereby controlling the amount of air-fuel mixture supplied to a combustion chamber. The power of the angina is controlled, accordingly. However, a high level of vacuum pressure (a low absolute pressure) is produced by the throttling action of the throttle valve. This increases pumping losses and thus reduces the engine efficiency.
To attempt to solve this problem, stratified charge combustion has been proposed. In stratified charge combustion, a relatively rich air-fuel mixture is delivered to the vicinity of an ignition plug for ensuring the ignition of the mixture, and the generated flame of the mixture burns the surrounding leaner mixture. The power of the engine is basically controlled by changing the amount of fuel injected to the vicinity of the ignition plug. This eliminates the necessity for changing the cross-sectional area of the intake passage to control the engine power, which decreases pumping loss and improves the engine efficiency. Further, stratified charge combustion allows an engine to be operated with a relatively lean air-fuel ratio and thus improves the fuel economy of the engine.
Engines that switch between stratified charge combustion and homogeneous charge combustion in accordance with their running state have been proposed. One such engine has a homogeneous charge fuel injection valve and a stratified charge fuel injection valve. The homogeneous charge injection valve uniformly disperses fuel into the combustion chamber and the stratified charge injection valve injects fuel toward the vicinity of the ignition plug. Another type has a fuel injection valve that directly injects fuel into the combustion chamber and switches between stratified charge combustion and homogeneous charge combustion. Switching to stratified charge combustion from homogeneous charge combustion when the engine load is small improves the engine efficiency and the fuel economy.
Some engines are provided with a brake booster that increases the braking force thereby decreasing the force required to depress the brake pedal. The brake booster uses vacuum pressure, which is produced in the intake passage downstream of the throttle valve, as a drive source. That is, vacuum pressure is communicated with the brake booster through a communicating pipe connected to the downstream side of the throttle valve. Vacuum pressure, which corresponds to the degree of depression of the brake pedal, acts on a diaphragm incorporated in the brake booster and increases the force actuating the brake.
In such an engine, pressure in the intake passage is decreased during homogeneous charge combustion. That is, the vacuum pressure available for actuating the brake booster is sufficiently low. However, pressure in the intake passage is increased during stratified charge combustion. That is, there is less vacuum available. This may result in insufficient vacuum to actuate the brake booster. As a result, the force required to depress the brake pedal is not decreased.
Japanese Unexamined Patent Publication No. 8-164840 discloses an apparatus for controlling vacuum pressure in internal combustion engines for solving this problem. In this apparatus, a throttle valve in an intake passage is closed when the pressure in a brake booster is greater than a predetermined level thereby temporarily lowering the intake vacuum pressure. Accordingly, a sufficiently low vacuum pressure for actuating the brake booster is communicated with the booster.
Incidentally, some engines are provided with an exhaust gas recirculation (EGR) mechanism for reducing exhaust emissions of nitrogen oxide (NOx). An engine having this mechanism includes a passage that communicates the intake duct with an exhaust duct. The passage recirculates some of exhaust gas in the exhaust duct to the intake duct. A flow control valve is located in the passage. The flow valve is controlled by an electronic control unit of the engine to adjust the amount of exhaust gas recirculated to the intake duct. Supplying inert gas, that is, exhaust gas, to intake air in the intake duct lowers the combustion temperature in the combustion chamber thereby reducing the exhaust emission of NOx.
If provided in an engine having the EGR mechanism, the vacuum pressure controlling apparatus of the above publication causes the following problems. In the vacuum pressure controlling apparatus, the throttle valve is closed for producing sufficient vacuum for actuating the brake booster. Closing the throttle valve simultaneously with is recirculation of exhaust gas by the EGR mechanism increases the ratio of recirculated gas in the air drawn into the combustion chamber. Accordingly, the amount of usable intake air supplied to the combustion chamber is abruptly decreased. This temporarily causes the air-fuel ratio in the vicinity of the ignition plug to be excessively rich. The overly rich air-fuel ratio causes misfires in the engine thereby fluctuating the torque of the engine.